Generally, the spread-spectrum communications such as CDMA (Code Division Multiple Access) performs a plurality of modulations at a transmitting side, and corresponding demodulations at a receiving side, thereby communicating information symbols. More specifically, the transmitting side transmits information symbols after performing a primary modulation, such as PSK, of the information symbols, and then, spreading the primary modulation signal into a wideband frequency range by performing a secondary modulation using a spreading code such as a high-rate pseudo-random code. On the other hand, the receiving side performs despreading (secondary demodulation) using the same and synchronized spreading code with the transmitting side so that the wideband frequency received signal is inversely converted into the band of the information symbols, and then, carries out a primary demodulation corresponding to the primary modulation, thereby restoring the original information symbols.
When such a spread-spectrum communication method is employed in mobile communications, a signal transmitted from a base station or a mobile station will reflect from obstacles such as buildings or the like on its propagation paths. Accordingly, the signal is received as a multipath signal whose component waves (delayed waves) arrive at different times because individual delayed waves have different delay times on the propagation paths. If the variance of the delay times of the propagation paths is greater than one element length of a spreading code (one chip interval), fluctuations of the individual delayed wave components which are extracted at every chip interval can be handled as non-correlated quantity. In other words, the amplitude and phase of each delayed wave component can be considered to change independently. As a result, the average received level will be improved by combining the independent delayed wave components after making their phases coherent, or by selecting a delayed wave component whose amplitude is maximum. This idea is implemented as a well-known RAKE reception, wherein improvement in transmission characteristics can be expected owing to the path diversity receiving effect.
FIG. 1 is a block diagram showing a conventional spread-spectrum communication receiver carrying out the path diversity reception (RAKE reception). In FIG. 1, the reference numerals 1-1-1-N designate correlators. Each correlator 1-k (k=1-N) receives a spread-spectrum signal 100, into which a pilot signal is inserted, and performs despreading of one of delayed waves using the same spreading code. Here, the pilot signal is a signal (called a sounder) measuring a transfer function of a propagation path. The output of the correlator 1-k is supplied to a detector 2-k which detects one of the delayed waves. The output of the detector 2-k is supplied to a weighting circuit 3-k and a power detector 4-k. The power detector 4-k detects power of the delayed wave, and makes it a coefficient of the weighting circuit 3-k. Respective weighted signals are combined by a combining circuit 5. The combined signal is sent to a symbol decision circuit 6 which decides the symbol. When the weighting is performed using all the outputs of the power detectors 4-k, a maximal ratio combining is achieved, whereas when the detected signal of the maximum power is selected, a selection combining is achieved
The conventional spread-spectrum communication system has the following drawbacks.
(1) In the above-mentioned arrangement which detects the respective delayed waves independently, the operation of the detectors will become unstable if the signal-to-noise power ratio (SNR) or the signal-to-interference power ratio (SIR) of the delayed waves is small. PA1 (2) In the spread-spectrum communications, since the SIR changes greatly for individual information symbols, it is difficult to achieve an optimum combination even if weighting based on the received power is performed. Accordingly, only insufficient diversity effect can be obtained. PA1 (3) A cellular mobile communication system covers a wide service area by locating a plurality radio base stations. In the service area, however, there are some areas as in tunnels where received signal strength is weak and communication quality becomes low. It is impossible to communicate in such places. Although it may possible to provide these dead zones with new base stations, this is not economical because equipment and scale of a base station are large. PA1 (4) Although differences between delay times of propagation paths must be greater than one chip interval to divide a multipath wave into delayed waves, these differences are not always obtained in all areas. For example, assuming that the bandwidth of the primary modulation is 16 kHz, and the bandwidth of the second modulation using a spreading code is 128 times greater than that, that is 2048 kHz, the resolving power of the delay is about 0.5 micro-seconds. Accordingly, the multipath wave cannot be divided into individual delayed waves when the delay time differences of the propagation paths are less than this value. As a result, fading will occur as in TDMA, and one of the distinctive features of the CDMA are lost. PA1 a first receiving antenna for receiving an electric wave from the base station; PA1 a first delay circuit for providing a received signal supplied from the first receiving antenna with a predetermined amount of delay; PA1 a first transmitting antenna for radiating an output of the first delay circuit to the mobile station; PA1 a second receiving antenna for receiving an electric wave from the mobile station; PA1 a second delay circuit for providing a received signal supplied from the second receiving antenna with a predetermined amount of delay; and PA1 a second transmitting antenna for radiating an output of the second delay circuit to the base station, PA1 wherein a delay time of the first delay circuit and a delay time of the second delay circuit are set at one chip interval of a spreading code or more. PA1 a first amplifier for amplifying the output of the first delay circuit, and for feeding it to the first transmitting antenna; and PA1 a second amplifier for amplifying the output of the second delay circuit, and for feeding it to the second transmitting antenna. PA1 a third receiving antenna for receiving an electric wave from the base station; PA1 first combining means for combining a received signal fed, from the third receiving antenna with the output of the first delay circuit, and for supplying a combined signal to the first amplifier; PA1 a fourth receiving antenna for receiving an electric wave from the mobile station; and PA1 second combining means for combining a received signal fed from the fourth receiving antenna with the output of the second delay circuit, and for supplying a combined signal to the second amplifier. PA1 a first amplifier for amplifying a received signal fed from the first receiving antenna, and for supplying an amplified signal to the first delay circuit; and PA1 a second amplifier for amplifying a received signal fed from the second receiving antenna, and for supplying an amplified signal to the second delay circuit. PA1 a third transmitting antenna for radiating an output of the first amplifier to the mobile station; and PA1 a fourth transmitting antenna for radiating an output of the second amplifier to the base station. PA1 a plurality of correlators for despreading individual delayed waves contained in a received spread-spectrum signal by using an identical spreading code; PA1 a plurality of detectors, each detects one of delayed wave components outputted from the correlators; PA1 a plurality of weighting circuits, each multiplies an output of one of the detectors by a weighting coefficient; PA1 a combining circuit for combining outputs of the weighting circuits; PA1 a symbol decision circuit for making symbol decision of an output of the combining circuit; PA1 estimation means for estimating a transfer function of a propagation path associated with each of the delayed wave components on the basis of an output of the symbol decision circuit and outputs of the correlators; and PA1 a weighting coefficient control circuit for estimating an amplitude of a desired wave component of each of the delayed wave components on the basis of each of the transfer functions estimated, and for generating the weighting coefficients based on the amplitudes. PA1 a plurality of multipliers, each of which multiplies the output of the symbol decision circuit by one of the transfer functions estimated; PA1 a plurality of subtracters for obtaining differences between the outputs of the correlators and outputs of the multipliers, respectively, and for producing the differences as estimated errors; and PA1 a calculation circuit for performing adaptive algorithm recursively estimating the transfer functions from the output of the symbol decision circuit and the estimated errors outputted from the subtracters. PA1 the repeater comprising: PA1 a first receiving antenna for receiving an electric wave from the base station; PA1 a first delay circuit for providing a received signal supplied from the first receiving antenna with a predetermined amount of delay; PA1 a first transmitting antenna for radiating an output of the first delay circuit to the mobile station; PA1 a second receiving antenna for receiving an electric wave from the mobile station; PA1 a second delay circuit for providing a received signal supplied from the second receiving antenna with a predetermined amount of delay; and PA1 a second transmitting antenna for radiating an output of the second delay circuit to the base station, PA1 wherein a delay time of the first delay circuit and a delay time of the second delay circuit are set at one chip interval of a spreading code or more, PA1 and the spread spectrum communication receiver comprising: